coal
DESCRIPTION
coal based poer plant coal cumbustion pptTRANSCRIPT
Pulveriser
Bag filter
Exhaust gas
cyclone
BURNERS
INDIRECT FIRING SYSTEM
The Process of Combustion involves three stages:
(a) Ignition
(b) Combustion Stability
(c) Completion of Combustion
In Combustion process, ignition occurs in vapour phase. Solid and liquid fuels get ignited from their vapours.
Combustion of fuels
Solid Fuel (Coal):
Volatile matter is released over a
temperature of 250 - 900 degree
Centigrade.
The volatile matter is first ignited.
The coal particle upon releasing the
volatile matter become a char. The char
slowly burns out.
Causes of Ash Deposition
• Reduced heat transfer,
• Physical damage to pressure parts
• Erosion and corrosion of pressure parts
• Slagging
• Fouling
Post -Combustion Performance due to Ash
• Dry Ash Disposal
• Fly Ash Disposal problems
Power station performance
• Capacity
• Heat rate
• Maintenance
• Availability
Burner Arrangement
In a tangentially fired boiler, four windboxes ) are arranged, one at each corner of the furnace.The coal, oil and gas burners are sandwiched between air compartments.
The fuel and combustion air streams from these compartments are directed tangentially to an imaginary circle at the centre of the furnace. This creates a turbulent vortex motion of the fuel, air and hot gases which promotes mixing, ignition energy availability and thus combustion efficiency.
Combustion Air Distribution
The Combustion air, referred to as Secondary Air, is provided from FD Fans. A portion of secondary air called `Fuel Air', is admitted immediately around the burners (annular space around the oil/gas burners) into the furnace. The rest of the secondary air , called 'Auxiliary Air' , is admitted through the auxiliary air nozzles and end air nozzles. The quantity of secondary air (fuel air + auxiliary air) is dictated by boiler load and controlled by FD Fan inlet guide vane regulation.The burners are tiltable +/- 30o about horizontal, simultaneously at all elevations and corners. This shifts the flame zone across the furnace height and enables control over steam temperature.
Emerging trends in Fuel firing system Design1. Burners shall be designed to operate on multiple fuels from LCV
gases , Liquid fuels to solid fuel or in combination of these fuels .
2. Less polluting burners - Technology development
3 .Larger guarantee periods for high ash coals - Material selection,
Improved design features
4. New gadgets like thermal analysers, on line coal flow
measurement and control thro coal air dampers etc.,
5. Micro processor based on line measurement and control of
secondary air admission to individual burners.
6. Computer simulations using software tools viz., ANSYS,
CAEPIPE, STADD and CFD.
7. Virtual assembly using CAD tools in design office before taking
the product for production
COAL - AIR BALANCING IN FUEL PIPING
Un balance in Coal- air flow in to the furnace results in uneven heat release , uneven distribution of excess air and unpredictable Nox formation
1. Selection of Orifices for Coal air two phase flow regime.
2. Provision of on -line adjustment dampers in coal air flow path.
3.On-line measurement of coal-air flow using microwave techniques
4. Continuous measurement using dirty pitot- tube
OVERFIRE AIR
Overfire air is introduced into the furnace tangentially through two additional air compartments, termed as overfire air ports, designed as vertical extensions of the corner windboxes. The overfire air ports are sized to handle 15 percent of total windbox air flow.
The proven success of overfire air as a supplement to the tangential firing in limiting NOx formation during coal combustion lies in the fact that this technique inhibits formation of both fuel NOx and thermal NOx as an oxygen deficient environment is established in the primary combustion zone.
At design levels of overfire, a 20 to 30% reduction in NOx formation is achieved.
NOX DEPENDENCE ON OVERFIRE AIR FLOW
0
50
100
150
200
250
300
350
0 2 4 6 8 10 12 14 16
OVERFIRE AIR(%)
NO
(P
PM
@ 3
% O
2)
OFA VS NO
EXCESS O2 (%) VS NO
500 MW wind Box assembly at shop
Thank you all